How can I get Apache to accept a domain with an umlaut in a virtual host?

I have a virtual host setup for a site which has about 8 derivatives of a domain pointing at it. All the normal domains (without any special characters) work fine, but the domains which have ’ä’ characters in them don’t work.. Apache just sends those domains to the default Apache vhost.

What do I need to do in order to get this to work? I’m using apache 2 and everything I can find on Google makes me thing this should just work out of the box. I keep finding stuff about ‘AddDefaultCharset’ but that seems to be for the serving of files rather than the interpretation of the vhost itself.

Any ideas?